This paper describes the conclusions that can be drawn from the data taken thus far with the PHOBOS detector at RHIC. In the most central Au+Au collisions at the highest beam energy, evidence is found for the formation of a very high energy density system whose description in terms of simple hadronic degrees of freedom is inappropriate. Furthermore, the constituents of this novel system are found to undergo a significant level of interaction. The properties of particle production at RHIC energies are shown to follow a number of simple scaling behaviors, some of which continue trends found at lower energies or in simpler systems. As a function of centrality, the total number of charged particles scales with the number of participating nucleons. When comparing Au+Au at different centralities, the dependence of the yield on the number of participants at higher p T (∼4 GeV/c) is very similar to that at low transverse momentum. The measured values of charged particle pseudorapidity density and elliptic flow were found to be independent of energy over a broad range of pseudorapidities when effectively viewed in the rest frame of one of the colliding nuclei, a property we describe as "extended longitudinal scaling". Finally, the centrality and energy dependences of several observables were found to factorize to a surprising degree.

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This paper presents measurements of the elliptic flow of charged particles as a function of pseudorapidity and centrality from Cu-Cu collisions at 62.4 and 200 GeV using the PHOBOS detector at the Relativistic Heavy Ion Collider (RHIC). The elliptic flow in Cu-Cu collisions is found to be significant even for the most central events. For comparison with the Au-Au results, it is found that the detailed way in which the collision geometry (eccentricity) is estimated is of critical importance when scaling out system-size effects. A new form of eccentricity, called the participant eccentricity, is introduced which yields a scaled elliptic flow in the Cu-Cu system that has the same relative magnitude and qualitative features as that in the Au-Au system.

Pseudorapidity distributions of charged particles emitted in Au + Au, Cu + Cu, d + Au, and p + p collisions over a wide energy range have been measured using the PHOBOS detector at RHIC. The centrality dependence of both the charged particle distributions and the multiplicity at midrapidity were measured. Pseudorapidity distributions of charged particles emitted with |η| < 5.4, which account for between 95% and 99% of the total charged-particle emission associated with collision participants, are presented for different collision centralities. Both the midrapidity density, dN ch /dη, and the total charged-particle multiplicity, N ch , are found to factorize into a product of independent functions of collision energy, √ s N N , and centrality given in terms of the number of nucleons participating in the collision, Npart. The total charged particle multiplicity, observed in these experiments and those at lower energies, assumes a linear dependence of (ln s N N ) 2 over the full range of collision energy of √ s N N =2.7-200 GeV.